The present invention is applicable to any form of ventilation in which respiratory flow is used for cycling, although it is primarily useful in noninvasive pressure-support ventilation. The invention also finds use in invasive ventilation, especially of the pressure support variety. The invention finds greatest use in the presence of higher levels of pressure support, with patients who typically do not have sleep apnea but do have respiratory insufficiency or failure, where ventilatory assistance is typically provided mainly at night but may well be provided during some of the daytime. Cycling becomes a prominent issue when respiratory mechanics are abnormal, especially in chronic obstructive pulmonary disease (COPD).
In a ventilator, it is often necessary to have a control process to detect when the patient's actual respiratory cycle switches from inspiration to expiration or vice versa so that the ventilator can implement an appropriate ventilation response. For example, some ventilatory devices provide a process for determining when to trigger an inspiratory pressure for purposes of delivering an appropriate pressure during the inspiratory portion of the patient's respiratory cycle. Similarly, such a device may also have a process for determining when to cycle to an expiratory pressure for purposes of delivery of an appropriate machine-patient response during the patient's expiration. Such processes serve to synchronize the ventilator with the patient's actual respiratory cycle. Those skilled in the art will recognize that “triggering” is the event associated with the initiation of the pressure levels intended for the patient's inspiration and “cycling” is the event associated with switching to the pressure levels intended for the patient's expiration.
For example, a bi-level ventilator provides a higher pressure level during the inspiratory portion of the patient's breathing cycle, a so-called IPAP, and a lower pressure level during the expiratory portion of the breathing cycle, a so-called EPAP. Traditionally, the switching may be accomplished by monitoring the respiratory flow or pressure and defining a threshold level, such as zero or a percentage of peak flow. When the measured respiratory flow value falls below the threshold, the device will deliver the EPAP. Another alternative to such switching may involve recorded respiration rates and the monitoring of elapsed time from the start of inspiration; the machine may switch to the expiratory portion of the respiratory cycle after reaching a time that is the expected time for the inspiratory portion of the respiratory cycle.
A goal of these processes for cycling is to make a ventilator device more comfortable for a user because if respiratory events are not properly synchronized, the device may be quite uncomfortable for a patient. A considerably more important goal is to optimize gas exchange and, especially in COPD with severe expiratory flow limitation, to prevent prolonged inspiratory times which lead to dynamic hyperinflation. Current methods for cycling can sometimes improperly detect expiration and result in an improper pressure change. For example, if the ventilator cycles into expiration too early, less support will be provided to the patient during inspiration when it is needed. Thus, there is need to improve such processes by minimizing improper synchronization.